FRESH-WATER BIOLOGY 



no (in) Post-abdomen not bilobed. . M acrothrix laticornis (Jurine) 1820. 



Form round-ovate. Valves crested, 

 the dorsal edge serrate with fine teeth. 

 Head evenly rounded. Labrum with 

 large triangular process. Antennule 

 broader distally; a setiferous projec- 

 tion on posterior margin near apex; 

 anterior margin with several fine in- 

 cisions and clusters or rows of hairs; 

 olfactory setae conspicuously unequal. 

 Post-abdomen with numerous fine 

 spines and hairs; anus terminal. 

 Claws small. Color grayish white or 

 yellowish. Length, $?, o-S-o-7 mm.; 

 $ , 0.3-0.4 mm. 



Widely distributed; found in all 

 parts of the country but nowhere very 

 FIG. 1 112. M acrothrix laticornis. abundant. 



O.I MM. 



in (no) Post-abdomen bilobed 112 



112 (113) Conspicuous fold or folds of shell of head at cervical sinus.- 



M acrothrix montana Birge 1904. 



Form ovoid. Head large; dorsal margin evenly rounded; 

 the shell extended into collar-like folds in front of cervical 

 sinus. Antennules stout, large, enlarged near apex, about 6 

 anterior cross-rows of hairs, and 3-4 stouter posterior setae; 

 olfactory setae unequal. Post-abdomen bilobed. Claws hardly 

 larger than spines. Color transparent, in preserved speci- 

 mens. $ unknown. Length, $ , ca. 0.55 mm. 



Colorado. 



FIG. 1113. M acrothrix montana. 



113 (112) No such folds. M acrothrix hirsuticornis Norman and Brady 1867. 



Form broadly ovate, not very different from M. laticornis. 

 Antennules broad, flat, bent, varying in form but always enlarged 

 distally; with 6-8 rows of stiff hairs on anterior side; sometimes 

 stout setae on posterior side; olfactory setae unequal. Post-abdo- 

 men large, broad, bilobed; pre-anal part not flattened nor with 

 projection for abdominal setae; numerous small spines and hairs 

 on both anal and pre-anal parts. $ unknown. Length, 9 > -55 

 mm. 



New England, Colorado. 



FIG. 1114. Macrothrix hirsuticornis. 



